The present invention relates to a method to determine a predefined switching moment of a switch as and a line terminator, a control network unit and a network unit realizing such a method and a tree-like optical network including such a line terminator, such a control means or such a network unit.
Such a line terminator, a control network unit and such a network unit are already known in the art, e.g. from "the published European patent application, published at 26.03.1997, with the title "Arrangement for amplifying and combining optical signals, and method for upstream transmission realized therewith", with publication number EP 0 765 045 A1". Therein, an optical amplifier combiner arrangement is described in a tree-like optical network. The tree-like optical network consists of the cascade connection of dedicated branches, the combiner arrangement and a common branch. The arrangement is coupled between a plurality of optical network units and an optical line terminator via one of the dedicated branches and the common branch respectively. The tree-like optical network enables upstream transmission of information signals from the optical network units to the optical line terminator. The optical amplifier combiner arrangement includes for each branch of the dedicated branches
an optical amplifier to amplify an upstream information signal with a gain value and to thereby generate an amplified information signal with a predetermined power level; PA1 an optical switch on/off coupled between the optical amplifier and an optical combiner to pass the amplified information signal when the information signal is present and to interrupt the branch when the information signal is not present. PA1 a) capturing downstream grant information by a control network unit which is coupled between the line terminator and the switch; and PA1 b) determining the approximated switching moment by the control network unit based upon the downstream grant information. PA1 c) upstream transmitting by one of the plurality of network units coupled to the dedicated branch a predefined bit pattern similar to the information signal, and when the predefined bit pattern is passing through the switch, thereby truncating the predefined bit pattern and providing a truncated bit pattern related to the approximated switching moment; and PA1 d) detecting by the line terminator reception of the truncated bit pattern; and PA1 e) determining by the line terminator upon reception of the truncated bit pattern a measure of how much the predefined bit pattern is truncated and providing thereby a truncation measure; and PA1 f) calculating in function of the truncation measure a calibration value for the approximated switching moment; and PA1 g) providing the calibration value to the control network unit in order to determine the switching moment in function of the calibration value and the approximated switching moment. PA1 determining by the line terminator upon detection of the truncated bit pattern a receiving time of the truncated bit pattern; and PA1 defining a receiving time reference by determining the receiving time of the truncated bit pattern at the line terminator in the event when the switch is continuously open whereby the truncated bit pattern substantially equals to the predefined bit pattern; and that the calibration value is calculated in function of the receiving time and the receiving time reference.
The optical combiner is included in the arrangement to combine all amplified information signals according to a multiple access technique for application to the optical line terminator.
In order to explain the object of the invention a possible way of determining the switching moment of each switch is explained in this further paragraph. The above mentioned document describes in column 2 from line 11 to line 25 a network which uses for the upstream transmission a time division multiple access technique. This technique makes use of special grant information in order to indicate to a network unit the application of an upstream timeslot. When a network unit receives from the line terminator a grant, it will wait for a certain delay before transmitting an upstream information signal. These delays are determined during a so called ranging procedure whereby an identical virtual distance is introduced for each network unit. This ranging procedure assures that when looking to the multiple access principle, each information signal transmitted by the different network units, arrives at the optical combiner in order to use the common branch, the one after the other following its dedicated timeslot. When each network unit introduces a correct delay there will be no collision at the optical combiner between the upstream information signals.
Just before being combined, each information signal has passed the switch of its dedicated branch. The switch passed the information signal when it was present and interrupted the branch afterwards when the information signal was not present anymore.
The control of the switches is realized by
With this special grant information the moment of transmitting upstream information for each network unit of each branch is determined and, associated therewith, also the moment of switching on and off for each switch of each dedicated branch in order to pass the transmitted upstream information.
However, due to the difference in distance of the control network terminator to the combiner and the different switches to the combiner, an uncertainty on the switching moment remains. Indeed, the control optical network terminator is enabled by capturing the special grant information to determine the moment when the information signal arrives at the combiner but the moment when the information signal arrives at the switch i.e. the switching moment of the switch is with this method only approximated due to this difference in optical delay and electrical delay.